Semiconductor device and method of forming the same

A semiconductor device and a method of forming the semiconductor device are provided. The semiconductor device includes a substrate, an interconnection structure, an oxide semiconductor (OS) transistor and a contact structure. The substrate has a first surface and a second surface opposite to the first surface. The interconnection structure is disposed on the first surface, and the oxide semiconductor (OS) transistor is disposed on the second surface. Also, the OS transistor includes a back gate disposed on the second surface of the substrate. The contact structure is formed between the OS transistor and the interconnection structure, and the contact structure is electrically connected to the back gate. The contact structure penetrates through the substrate for electrically connecting the interconnection structure to the OS transistor.